Many utilities now burn a variety of coals at their fossil plants. This practice is growing for several reasons, including (1) the need to lower SO2 emissions by burning low-sulfur coals and (2) the need to reduce fuel costs to enhance competitive position. Frequently, changes in types of coal have adverse affects on ESPs. Low-sulfur coals produce high resistivity ash that is difficult to collect in an electrostatic precipitator (the technology most commonly used to control particulate emissions from coal-fired power plants). Inexpensive coals are frequently variable in their properties and sometimes high in ash or low in sulfur or both. SO3 conditioned ash, before it enters a precipitator, can lower ash resistivity and improve ESP performance. In fact, this well established technology is used at several hundred plants both here and abroad to control fly ash resistivity from low-sulfur or variable-sulfur coals.
Ammonia conditioning can be combined with SO3 conditioning in difficult applications. Ammonia, injected at the proper rate, reacts with SO3 (really sulfuric acid vapor) in the flue gas to produce ammonium bisulfate. This compound is liquid at typical ESP operating temperatures and produces two beneficial effects. First, it co-precipitates with the ash and increases the cohesivity of the ash, thereby reducing ash re-entrainment. This effect lowers outlet emissions due to re-entrainment, a problem that is frequently significant in small ESPs. Secondly, the presence of the ammonia compound makes certain difficult ashes easier to condition. In this country, the combination of ash from an eastern, bituminous coal and a high ESP operating temperature produces a difficult SO3 conditioning situation that can be alleviated through the addition of ammonia. Ammonia conditioning systems are simple and reliable, but the controls for these systems are not sophisticated. It is important to add ammonia at the correct rate to ensure optimum ESP performance and minimum ammonia emissions.
For some fly ashes, dual conditioning with SO3 and NH3 can improve collection efficiency by reducing the amount of re-entrainment by producing a dust that agglomerates due to the formation of ammonium sulfate and ammonium bisulfate on the fly ash particles. Dual conditioning also increases the temperature at which SO3 conditioning for ash resistivity reduction is effective.
The injection of sulfuric acid into the flue gas stream from a coal-fired boiler is used to lower the resistivity of the fly ash from coal combustion to optimize the collection of fly ash in an electrostatic precipitator. The efficiency of collection of fly ash in electrostatic precipitators is proportional to the electrical power input to the precipitator. The power input to the precipitator is proportional to the current density at which the precipitator operates. The current density at which a precipitator can be operated is directly related to the dielectric strength of the dust layer divided by the resistivity of dust layer. In general, as the dust layer resistivity decreases, the operating current density for the precipitator increases. If the dust layer resistivity is too high, then the electrical potential across the dust layer increases to the point at which the dust layer electrically breaks down forming-so-called back corona. If the dust layer resistivity is too low, the electrostatic holding force to keep the dust layer collected on the precipitator collection plates is too low. In which case the collected dust will be in re-entrained in the flue gas stream increasing emissions and reducing the efficiency of the precipitator.
Though the invention is directed mainly at fly ash collection, it is also applicable to the control of excessive SO3 emissions from the conversion of SO2 to SO3 in a selective catalytic reactor (SCR) for NOx emission control. In the case of an SCR, ammonia is added after the air preheater in the correct portion to form ammonium bisulfate for collection in a precipitator or baghouse (fabric filter).
Prior Art Patents
Humbert (U.S. Pat. No. 3,523,407) teaches the addition of predetermined amounts of ammonia water into a particle-laden gas stream to produce ammonia bisulfate.
McKewen (U.S. Pat. No. 3,665,676) adds ammonium sulfate or ammonium bisulfate to a flue gas stream.
Kalka (U.S. Pat. No. 5,424,044) teaches a method of adding ammonia to flue gas; SO2 and SO3 may be present when the ammonia is added and ammonium sulfate is formed. Kalka does not teach the herein disclosed invention in that ammonium bisulfate is not disclosed as being formed. Nor is there recognition of the need to employ stoichometric amounts and nor is there recognition in Kalka that ammonium bisulfate treated fly ash will facilitate removal of fly ash from the plates of an electrostatic precipitator.
Wright (U.S. Pat. No. 5,665,142) teaches the use of ammonia in flue gas as a conditioning agent, however, the stoichometric relationship between ammonia and sulfur trioxide is not recognized, nor is the beneficial effect of reducing particle reentrainment.
Objects of Invention
An object of this invention is to produce a process wherein the removal of fly ash from flue gas is enhanced.
A further object of this invention is to enhance the effectiveness of SO3 for the removal of fly ash from flue gas.
A main object of this invention is to add optimum amounts of ammonia to flue gas containing fly ash to produce enhanced fly ash adhesiveness.
A further object of this invention is the addition of optimum amounts of sulfur trioxide (SO3) and ammonia to a flue gas stream, involving an electrostatic precipitator, to enhance fly ash separation from said flue gas.
These and other objects of the present invention will become apparent from a reading of the following specification.